It is important to know whether hemispheric asymmetries in cognitive processing are unique to human intelligence and consciousness or whether they represent more fundamental vertebrate adaptations. Moreover, if other animals do have human-like lateralization then the possibility exists of studying the neural mechanisms of hemispheric specialization with many modern techniques not appropriate for human experiments. In addition, cognitive and structural models of laterality may be evaluated meaningfully with data from animals as well as from human beings. Such research should have profound effects for understanding a variety of diseases, such as dyslexia, immune disorders, psychiatric disturbances, and hormonal imbalances, that are now being associated with atypical lateralization of the brain. Rhesus monkeys are being studied to see whether their left and right hemispheres differ in cognitive processing as do the hemispheres of human beings. This is done by independently testing the separated hemispheres of split-brain monkeys on a variety of visual discriminations that are known to show lateralized processing in human subjects. Key features of the discriminations are manipulated and the effects on lateralization noted. Such effects also are looked for when split-brain patients perform similar discriminations, allowing the similarity of hemispheric specialization in both species to be assessed. For example, it appears that both split-brain monkeys and human beings process facial information better with the right cerebral hemisphere than with the left. The effects of inverting facial stimuli on recognition are being studied because they permit specific models of facial processing to be tested. Once laterality has been measured for a variety of tasks, it should be possible to see how well performance in different tasks correlates, and then to infer if lateralization of function arises from fundamental hemispheric differences.